A hot gas heating device is described in DE-C-3 401 959 for heating the upper region of a plastic or laminate tube which is already closed at one end and filled with the product which it is to contain. The material of the upper inner wall region of the tube is heated to a temperature at which it becomes soft by means of hot gas exiting a heating nozzle placed within the upper wall region of the tube. To securely locate the tube during the heating phase, a centering sleeve having cooling means is provided in abutment with the outer upper region of the tube adjacent the heating nozzle. From the inner upper wall region which is to be the weld site, the hot gas flows into a collector housing from whence it is exhausted along a conduit. After the heating operation, the tube is transferred to a press station where the heated end region of the open tube is sealably closed by means of press jaws.
Whilst the above-described type of device has proved to be a popular alternative to conventional high frequency welding, contact heat jaw welding and radiation heat welding, it has until now not been possible to seal a tube in such a way that the exterior weld site is not visibly damaged by temperature fluctuations around the weld region. So as not to highlight the impaired finish of this region, preprinted tubes are used whose upper end region is kept free from print. Marketing forces have, however, created a need for a device which can successfully seal a tubular container without affecting the outer upper wall region of the tube in an unaesthetic manner, thereby allowing complete print coverage of the entire exterior surface of the tube.